1. Field of the Invention
The invention relates to radial-axial electromagnetic flux electric motors, coaxial electromagnetic flux electric motors, and rotors for a radial-axial electric motor.
2. Description of Related Art
Many high performance applications, such as electric actuators in aircraft and variable speed motor drives used in defense underwater applications, require highly compact, low mass, and low volume electrical machines. The power density of these high performance electrical machines may be quantified in units of Watt/lb, hp/lb, or kW/Kg to express the power density relative to weight and in units of W/in3 or hp/in3 or kW/m3, to express the machine's power density relative to its volume. When the electrical machine's power is expressed relative to its weight or volume, the relevant power density metric is more readily identifiable.
One way of increasing the power density of an electrical machine is to operate the machine at high speed. Since the machine's power is proportional to the speed at which it operates, higher operational speed provides higher machine power, so long as the flux in the machine is kept constant. Electrical machines with one stator and one rotor are also used to obtain high speed and, thereby, high power density. Additionally, high energy density and high flux density permanent magnets are used in electrical machines to improve their power densities. In all high density machines, though, the one stator and one rotor concept is widely applied.
High power density machines are mostly employed in applications requiting high reliability, such as is required in military and aircraft applications. Usually, related art high power density machines, such as permanent magnet synchronous machines (PMSMs) and permanent magnet brushless direct current (dc) machines (PMBDCMs), are not capable of providing high reliability under all operating conditions.
For instance, consider a machine experiencing a short circuit of one phase winding, while the machine is operating. The short circuit causes a large current to flow in the effected winding. Because the effected phase winding is mutually coupled with the other phase windings of the machine, the short circuit current also affects the operation of these other windings. As a result, these other windings are not able to operate in a normal manner, since they may require higher than nominal voltages to operate. The short circuit will bleed power from the source and put it to waste, resulting in low power efficiency. Therefore, such consequences are undesirable.
U.S. Pat. Nos. 3,396,296, 3,602,749, 3,729,642, and 4,114,057 disclose machines employing two stators and one rotor in the radial direction. Each of these patents describe a brush dc machine, with a commutator and two stators, that use permanent magnet excitation both in the radial and axial direction to maximize the induced electromotive force (emf) and, hence, the machine torque. The rotor has the windings and a commutator on its shaft. Since only one winding is used on the rotor to generate torque, the machine's ability to operate with a fault is low or non-existent. Though these patents describe techniques for increasing efficiency, they do little to advance the fault tolerance of the machines.
U.S. Pat. No. 6,373,160 discloses an electric machine with one stator winding and two separate rotors, whose rotational speeds may differ. The disclosed structure is achieved by extending the machine in the axial direction and diminishing the volume of space between the two rotor shafts, thus significantly reducing the power density of the machine. Also, a fault occurring in the stator winding prevents power from being applied to either rotor shaft. These shortcomings limit the applications to which this machine may be applied.
U.S. Pat. No. 6,497,201 discloses a switched reluctance machine architecture having two rotors and two stators. The two rotors are separate but concentrically disposed. The stators are disposed between the inner and outer rotors and are concentrically placed with respect to the rotors. A first rotor and stator pair constitutes a first electrical motor and a second rotor stator pair constitutes a separate, second electrical motor. These two motors are disposed concentrically about one another. The disadvantage of this motor architecture is that it increases the rotor inertia, which is proportional to the fourth power of the diameter. The increased rotor inertia reduces the acceleration of the machine shaft and decreases the dynamic response of the machine to load changes. Also, this architecture does not advance the art in terms of arranging the relative placement of the motors, arranging the stator windings of the motors, and the magnetic arrangement of the rotors.
All reference material cited herein is hereby incorporated into this disclosure by reference.